1. Field of the Invention
The present invention relates generally to instruments and methods for the non-invasive quantitative measurement of blood glucose and, more specifically, to a procedure for verifying the accuracy of non-invasive glucose measurement instruments.
2. Description of Background Art
Various instruments and devices exist which provide for non-invasive measurement of blood glucose. Such devices utilize different technologies to measure glucose content, including RF energy passed through the finger, longer wavelength near-infrared energy used in the reflectance or trans-reflectance mode, light energy used to measure the fluid of the eye, an optical technique used to look through the eye at the capillaries in the back of the eye, the use of sweat, blood sucked through the skin, and many others. For a disclosure of non-invasive blood glucose instruments which use near-infrared energy to measure glucose content, as well as means for custom calibrating the non-invasive instruments, see U.S. Pat. Nos. 5,028,787, 5,068,536, 5,077,476, 5,086,229 and 5,204,532, the contents of all being incorporated by reference herein.
Commercial in vitro (invasive) measurement instruments, i.e., those which require a drop of blood to measure blood glucose content, verify their own accuracy and stability by a combination of two procedures. First, these instruments include a special colored piece of paper or plastic that acts as an optical standard. This optical standard is periodically measured by the instrument to ensure that the instrument's calibration is correct and it is providing stable readings.
Second, a special glucose solution is provided for use with the in vitro instruments. The glucose solution mimics blood glucose and is used in combination with the instrument's chemically-laden strips to show that the strips and instrument react properly to the presence of glucose.
A similar paper/plastic optical standard procedure can also be used for calibration of non-invasive glucose measurement instruments, for example, to show that the non-invasive instrument remains stable over time by periodically measuring the standard with the instrument. However, it may be necessary in some circumstances to provide more long-term evidence that the non-invasive instrument remains stable. For example, such circumstances may include regulatory requirements or requirements of a research protocol.
Testing the accuracy of the non-invasive instrument is more difficult if the non-invasive instrument is custom calibrated, i.e., if it uses an individualized calibration that applies to a particular person. In this case, it is difficult to verify that the non-invasive instrument is providing accurate readings if the individual's body composition changes, e.g., due to a body fat increase, or if the person starts taking a new medication.
Accordingly, there is a need in the art for a procedure that verifies the accuracy of non-invasive blood glucose measurement instruments.